Download sizes not accurate?

When choosing to download a program onto my phone, it lets me select the quality of low, medium and high. However, when I choose low, it states the file size will be 173 megs. When it starts downloading, it now says X MB of 260 MB. Obviously larger than 173. I look a minute later, and now it says X MB of 320. File size grew, and it's not the original 173.